Just know that if you've neglected to shake your cartridge regularly (Roland recommends daily) then the ink will have settled in the cartridge and regular cleanings will have filled the ink lines with pigment-less (or at least less pigment than is expected) ink. After getting the pigments back into suspension in the cartridge, it will take at least a couple of powerful cleanings to prime the lines. Then I would print a large white block to evaluate the white coverage.
Then, make sure that you're evaluating the print under the circumstances that it will be used. If you're going to put it on a window, mount it there temporarily to see how it appears. The white will probably not be opaque even at its best, but will be sufficient to provide a background for your signage. And OK signs is right, you'll probably want to lay down a couple of layers of white to get the density needed for most jobs. I usually print white only, return to origin then print white>CMYK on top of that. (Or reverse if second surface)
